In Apple Configurator or Profile Manager, open the iOS Restrictions payload window.

Deselect "Allow installing configuration profiles (supervised only)."

If you use a third-party MDM solution, look for the key "allowUIConfigurationProfileInstallation".

When you set this restriction, users can't install profiles from email messages or websites. This includes the profile that enrolls the device in the iOS Beta Program. You can still use Apple Configurator or MDM to install profiles on the devices you manage.